diff --git a/src/thermo.cpp b/src/thermo.cpp
index 31c19fbdc2..1c07da695f 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -201,6 +201,8 @@ void Thermo::init()
   ValueTokenizer *format_line = nullptr;
   if (format_line_user.size()) format_line = new ValueTokenizer(format_line_user);
 
+  field_data.clear();
+  field_data.resize(nfield);
   std::string format_this, format_line_user_def;
   for (int i = 0; i < nfield; i++) {
 
@@ -208,6 +210,14 @@ void Thermo::init()
     format_this.clear();
     format_line_user_def.clear();
 
+    if (vtype[i] == FLOAT) {
+      field_data[i] = (double) 0.0;
+    } else if (vtype[i] == INT) {
+      field_data[i] = (int) 0;
+    } else if (vtype[i] == BIGINT) {
+      field_data[i] = (bigint) 0;
+    }
+
     if ((lineflag == MULTILINE) && ((i % 3) == 0)) format[i] += "\n";
     if ((lineflag == YAMLLINE) && (i == 0)) format[i] += "  - [";
     if (format_line) format_line_user_def = format_line->next_string();
diff --git a/src/thermo.h b/src/thermo.h
index 8a5cba29d7..f22f3103b0 100644
--- a/src/thermo.h
+++ b/src/thermo.h
@@ -43,6 +43,7 @@ class Thermo : protected Pointers {
   int evaluate_keyword(const std::string &, double *);
 
   // for accessing cached thermo data
+  int get_nfield() const { return nfield; }
   bigint get_timestep() const { return ntimestep; }
   const std::vector<multitype> &get_fields() const { return field_data; }
   const std::vector<std::string> &get_keywords() const { return keyword; }